High Ductility, Shear-Controlled Rods for Concrete Reinforcement

ABSTRACT

A re-bar (reinforcing rod) for concrete comprises an inner rod of a first material, and an over-wrap of a second material. The over-wrap may be structurally or functionally discontinuous relative to the inner rod.

The present invention relates to the field of concrete reinforcement,and in particular provides pseudo-ductile polymer-based (monolithicpolymer or Fibre Reinforced Polymer, FRP) re-bar rods of several noveldesigns. Each utilizes controlled and predictable interfacial frictionduring the relative sliding of elements of the re-bar as a means toinduce pseudo-ductile behaviour in the re-bar.

Traditionally, the material of choice to reinforce concrete has beensteel, in the form of rigid re-bar rods, flexible grids, wire, or pre-or post-tensioned wires and cables.

Steel reinforced concrete is a composite material that combines thepositive attributes of both constituents, steel and concrete, andresults in a composite that is superior to both. Concrete is ananisotropic material that has the quality of low cost (production andtransportation cost) and a very high compressive load carrying capacity.Its ultimate compressive strength ranges between 40 MPa for general useconcrete to about 90 MPa for high strength concrete. Under controlledlab environments even higher strength may be achieved. The majordrawback of concrete is its very low tensile load carrying capacity. Thetensile strength of concrete is only about 10% of its compressivestrength. In order to counteract this drawback, steel reinforcingmembers capable of carrying high tensile loads, generally in the form ofre-bar rods, are inserted along the tension side of a concrete member.In order to increase the bond strength between the steel rods and theconcrete, the rods are manufactured with a high surface roughness, themost common being in the form of spaced rings or spiralling protrusionsalong their length.

The tensile strength (yield) of steel is about 10 times that of concrete(ultimate strength). As a result the amount of steel reinforcementrequired along the tension side of concrete members is not great, andthe cost of that reinforcement is an insignificant fraction of the totalcost of a project. Steel's most important characteristic as areinforcement material is its purely plastic behaviour beyond the yieldpoint. Between this point and failure, elongation of up to 40% at arelatively constant stress level provides its high-ductility. Thisbehaviour produces very noticeable cracks in concrete structures as theybegin to fail and is an essential life saving characteristic; the earlywarning allows for evacuation of the structure before complete failure.

Steel, however, has the major drawback of susceptibility to rustparticularly in salty or chemically lathed environments. Sea shorestructures, and those in cities where salt or chemicals are used to dealwith ice and snow accumulation on roads, bridges and garages are typicalstructures that suffer from such a problem. The cost of repairs ofrusted reinforcement in concrete structures is very high and repairs arequite disruptive. As an alternative to steel, polymer-based solutionshave been considered. One possible solution is to use a monolithicpolymer rod whose elastic modulus and yield strength match that ofsteel. At present there are no polymers that have achieved these valuesin rod similar in diameter to that of existing steel rebars. There arecontinuing improvements in the mechanical properties of such largediameter rods (recently the elastic modulus of polyethylene rods hasincreased form 1.0 to 20 GPa as a result of new processing techniques).It is quite conceivable that additional changes in processing couldincrease the elastic modulus of polymeric rods to match that of steel,i.e., 200 GPa. In the meantime major research efforts have identifiedFibre Reinforced Polymers (FRP) as candidate materials for re-bars. Atpresent, carbon, aramid, and glass fibres are typically used toreinforce a polymer matrix to form the re-bars.

One of the major advantages of using fibre composites as material forcomponents is. their design flexibility. In the most general sense thismeans that a designer may take advantage of the high strength, highmodulus reinforcing fibres by aligning them in the matrix along theprincipal stress directions. Since re-bars in concrete are located totake primarily tensile load, fibres in FRP re-bars are aligned along thesingle principal stress direction, the longitudinal axis, of the re-bar.

While FRP re-bars can match the strength, modulus, and concrete/re-barbonding requirements, however, they suffer from a lack of ductility (%elongation at failure). This would also be true for the monolithicpolymer rods described previously.

Due to the absence of FRP re-bars with adequate ductility, one newapproach by others for the design of concrete structures is beingdeveloped. In this approach, the FRP re-bars which are the tensile forcecarriers are over-designed by applying an excessively large factor ofsafety to the ultimate strength and changing the initial failurecriterion to concrete crushing in the compressive region. This approachis costly, and accordingly, an aim of the present invention is todevelop FRP re-bars with mechanical properties that are similar to thoseof traditional steel re-bars. In this case the design approach (andcodes) would not need to be changed.

Several research publications and patents dealing with FRP re-barductility issues have appeared over the past few years. The most commonapproach used to produce high “ductility” FRP re-bars whosestress-strain behaviour matches that of steel is to manufacture a hybridFRP rod using several types of fibre with varying strength and strain tofailure values.

The first such endeavour is attributed to Bunsell and Harris where intheir 1974 publication “Hybrid Carbon and Glass Fibre Composites” theydemonstrated “pseudo ductility” characteristics for a hybrid bar made ofalternating laminates of glass and carbon fibres. In general, hybrid FRPre-bars are currently made using three types of fibre. Carbon fibres arealmost always used to provide the elastic modulus equal to that ofsteel. E-Glass fibres are commonly used to provide the ductility. Aramidfibres, such as Kevlar, are also used as a third fibre type that has amodulus in-between the moduli of carbon and glass and a strain tofailure greater than that of glass fibres. As a hybrid re-bar is loaded,the carbon fibres fail first between 0.2 and 2% strain, the load istransferred to the glass fibres which eventually fail at about 2.4%strain, where upon the load is transferred to the aramid fibres andresults in a total strain to failure of the FRP re-bar of about 3.5%.The characteristics of these fibres together with those of steel andconcrete in tension are shown in FIG. 1. Appropriate amounts of thedifferent fibres are used in the composite re-bar so as to achieve therequired strength, modulus, and relatively constant stress up tofailure. Unfortunately, the maximum ductility is limited to the highestultimate failure strain of the selected fibres, typically 3.5%. Atypical stress-strain plot of hybrid re-bars reported in De la Rosa,César, “Length Effect in Hybrid FRP Re-bars for Reinforced ConcreteApplications”, M. Eng. Thesis, Mechanical Engineering, University ofOttawa, August 2002, is shown in FIG. 2. This approach was initiallyproposed in 1996 by Arumugasaamy and Greenwood and patented in 1998,U.S. Pat. No. 5,727,357. Several researchers have investigated thisapproach since then, including Manis, P. A., “Manufacture -andperformance evaluation of FRP re-bar featuring ductility”, M.S. Thesis,University of Missouri-Rolla, 1998, 77 pages; Somboonsong, W., Ko, F.K., and Harris H. G., “Ductile Hybrid Fibre Reinforced PlasticReinforcing Bar for Concrete Structures: Design Methodology”, ACIMaterials Journals, V95, No. 6, 1998 655-666.

A second approach for high-ductility FRP rebars was that proposed byU.S. Pat. No. 6,071,613 (Rieder et al) among others. Their approacheswere to increase the toughness of the concrete itself (without re-bars)by using short, discontinuous, randomly oriented fibres to control thebehaviour at crack openings.

A further approach involves orienting continuous fibres at an angle tothe longitudinal axis of the re-bar. The fibres can be oriented at anangle to the longitudinal axis of the re-bar by processes, such as 2Dbraiding and filament winding, see, eg. Somboonsong (above); Belardi Ai,Chandrashekara K., Watkings, S. E., “Performance Evaluation of FibreReinforced Polymer Reinforcing Bar Featuring Ductility and HealthMonitoring Capability”; and Belbardi A., Watkings, S. E.,Chandrashekara, K., Corra, J., Konz, B. “Smart fibre-reinforced polymerrods featuring improved ductility and health monitoring capabilities”,Smart Materials and Structures Vol. 10, 2001, 427-431. For these designsductility is achieved by the re-orientation of the angled fibres underload. This approach was proved to be unsuccessful as the maximum failurestrain achieved was 2.1% due to the limited change in the length as thefibres are re-oriented.

Edwards and D'hooghe in Canadian Patent No. 2,396,808 proposes the useof composite material where the matrix is thermoplastic-in order to takeadvantage of the its flexibility, particularly as it is heated. In oneof its embodiments they propose the use of short fibres in thethermoplastic matrix as the core of their re-bar.

The use of short fibres is also done in traditional fibre composites inwhich increased toughness and ductility is achieved by the pullout ofthe short fibres from a matrix. The frictional shear stress that existsbetween the fibres and the matrix can support a tensile load at the sametime. In theory this approach would lead to ductilities of up to 50% ifthe pullout mechanism was equally distributed along the length of theFRP re-bar. However, it is extremely difficult to ensure uniformalignment, uniform bonding, uniform spacing, etc. at themicro-structural level. In addition, there is a very wide range ofultimate tensile strengths of the fibres as found in any high strength,brittle materials. Because of this lack of uniformity, a failureinitiates at a local non-uniform point and failure propagates from thispoint. Increased ductility is achieved only in that small local region.

It is understood that the concept of the pullout of fibres can besuccessful in increasing ductility if pullout can occur uniformly. Thisrequires that the reinforcing elements have uniform strengths, bondstrengths during pullout, uniform alignment, uniform spacing, etc. TheApplicants have discovered that uniform strength during pullout can beachieved by ensuring that the frictional shear stress between slidingelements is controlled. This sliding may occur between individual dowels(meso-rods) and matrix or between an inner rod and an over-wrap. It isessential that the sliding occur along the length of the re-bar. Forre-bar made with discontinuous meso-rods in a polymer matrix, thisrequires uniform reinforcement (at each cross-section) along the lengthof the re-bar. The length of the meso-rods must also be less than acritical length, L_(c) _(m) otherwise tensile failure of the meso-rodwill occur rather than the sliding at the interface.

$\begin{matrix}{L_{c_{m}} = \frac{\sigma_{um}r_{m}}{\tau_{m}}} & (1)\end{matrix}$

where L_(c) _(m) is the critical length of the meso-rod

-   -   σ_(um) is the ultimate tensile strength of the meso-rod    -   r_(m) is the radius of the meso-rod, and    -   τ_(m) is the frictional shear stress between a meso-rod and the        surrounding matrix        For the over-wrap case, sliding can be achieved by having the        over-wrap discontinuous with the discontinuous lengths less than        the critical length for the inner rod/over-wrap system:

$\begin{matrix}{L_{c_{o}} = \frac{\sigma_{ur}r_{r}}{\tau_{r}}} & (2)\end{matrix}$

where L_(c) _(o) is the critical length of the over-wrap

-   -   σ_(ur) is the ultimate tensile strength of the inner rod, and    -   τ_(r) is the frictional shear stress between the inner rod and        the over-wrap.

In one broad aspect the present invention relates to a reinforcing rodcomprising an inner rod of a first material, and an outer over-wrap of asecond material, said over-wrap being structurally discontinuousrelative to said inner rod.

The inner rod can be made from a monolithic polymeric material or afibre composite material consisting of fibres and a polymeric matrix.The outer layer is preferably an over-wrap of a fibrous material set ina polymeric resin matrix. The fibrous material is selected from thegroup consisting of ceramic materials including carbon fibres, glassfibres, particularly E-glass fibres and the group of polymeric fibres,such as aramid fibres and polyethylene fibres. Metallic fibres may alsobe used. The resin may be selected from the group of thermosettingresins such as epoxies, polyesters, and vinyl esters, and vinyl estersand/or thermoplastic resins, such as nylon or polyethylene andpolypropylene.

The structural discontinuity of the over-wrap is defined by zones ofweakness separating full strength lengths of the over-wrap. That is, thezones of weakness may be formed by mechanically removing a portion ofthe second layer after it has been applied to the inner rod. However,the zones of weakness may be achieved by short, spaced apart lengths ofsaid inner rod having no over wrap over same.

A zone of weakness may also be introduced in a continuos over-wrap usingannular sections of a low coefficient of friction material (for example,polytetraflouroethylene) that is placed around the inner rod at variouspoints along the inner rod (FIG. 3 b). At any cross-section of there-bar, the tensile load is being carried by the inner rod (in tension)and the over-wrap in shear at the interface between the over-wrap andthe inner rod. Since minimal shear load transfer will occur in theportions with the low friction material, the load normally carried inshear at the interface will be transferred to the over-wrap as anincreased tensile load. This will result in tensile failure of theover-wrap, i.e., a zone of weakness.

In a preferred embodiment, the inner rod is a cylinder having radiusr_(r) and an ultimate tensile strength σ_(ur). The frictional shearstress after original bond failure between the inner rod and theover-wrap is τ_(r), and the over-wrap is comprised of structurallydiscontinuous portions having a maximum length L_(c) _(o) , wherein

$\begin{matrix}{L_{c_{o}} \leq \frac{\sigma_{ur}r_{r}}{\tau_{r}}} & (3)\end{matrix}$

Preferably, said radius r is in the range of 1-30 mm and said lengthL_(c) _(o) is in the range of 1-150 cm.

More preferably, radius r is in the range of 3-8 mm.

More preferably, radius r is in the range of 4 -6 mm.

Optimally, radius r is in the range of 4 -5 mm.

A functionally determined radius r is 4.5 mm.

The length L_(c) _(o) may be in the range of 10-20 cm.

Moreover, length L_(c) _(o) is preferably in the range of 12-18 cm.

A functionally determined length L_(c) _(o) is about 15 cm.

In another broad aspect, the present invention relates to a method ofinducing pseudo-ductility in a fibre reinforced composite inner rod,said inner rod comprising a solid core and a fibre reinforced polymericresin over-wrap on said core, said method comprising structurallyinterrupting said over-wrap at spaced apart locations. The over-wrap maybe applied as a resin impregnated fibre braid.

A reinforcing rod comprising a composite rod having an inner core and anouter surface, said outer surface being textured over a predeterminedportion thereof to mechanically grip a concrete matrix in which a saidrod is embedded.

The over-wrap may be applied as a resin impregnated fibre yarn,unidirectional tape or woven fabric tape helically wound on said core.

Advantageously, the over-wrap is structurally interrupted by being cutin spaced apart annular rings or a continuous helical pattern.

The method of the present invention comprises the steps of i) providingan inner rod comprising solid core of a monolithic polymer or a fibrereinforced polymer; ii) applying bands of material having low frictionalshear stress at spaced apart locations on said solid core; iii) applyinga fibre reinforced polymeric resin over-wrap over the banded core,whereby said bands of low frictional shear stress material structurallyseparate zones of over-wrap bonded to said core.

In the method of the present invention, the inner rod is preferably acylindrical rod having radius r_(r) and an ultimate tensile strengthσ_(ur), the frictional shear stress after bond failure between the innerrod and the over-wrap is τ_(r), and said over-wrap is comprised ofstructurally discontinuous portions having a maximum length L_(c) _(o) ,wherein

$L_{c_{o}} = \frac{\sigma_{ur}r_{r}}{\tau_{r}}$

In an advantageous embodiment, the re-bar comprises at least threematerials, at least two of which are present in structurallydiscontinuous lengths. The composite may comprise a polymer matrixhaving embedded therein structurally discrete meso-rods of length L_(c)_(m) with radius r_(m), ultimate and tensile strength σ_(um), thefrictional shear strength between a meso-rod and the polymer matrixbeing represented by τ_(m), wherein

$\begin{matrix}{L_{c_{m}} \leq \frac{\sigma_{um}r_{m}}{\tau_{m}}} & (4)\end{matrix}$

Moreover, the structurally discrete meso-rods preferably comprise aplurality of meso-rods each with a radius less than half that of thecomposite rod. The structurally discrete dowels may comprise a pluralityof elongate meso-rods breakable by a tensile load substantially lessthan the ultimate tensile strength of each meso-rod, at predeterminedweakened locations along the dowels.

It will be understood that the ends of the discrete meso-rods, or thepredetermined weakened points in the elongate meso-rods will be randomlydistributed, so that several meso-rods do not end at the same point,which would lead to a weak, relatively unreinforced area of matrix.

L_(c) _(m) is preferably in the range of 5-30 cm.

L_(c) _(m) is more preferably in the range of 5-25 cm.

L_(c) _(m) is even more preferably in the range of 8-20 cm.

L_(c) _(m) is yet more preferably in the range of 10-15 cm.

L_(c) _(m) is most preferably in the range of 11-13 cm.

L_(c) _(m) is optimally about 12 cm.

r_(m) is preferably in the range of 0.5-4.0 mm.

r_(m) is more preferably in the range of 0.5-3.0 mm.

r_(m) is even more preferably in the range of 1.0-3.0 mm.

r_(m) is most preferably in the range of 1.5-2.5 mm.

r_(m) is optimally about 2.0 mm.

The meso-rods may be made from a material selected from the groupconsisting of ceramic materials including carbon fibres and glassfibres.

The polymer matrix may be selected from the group consisting ofthermoset resins including epoxies, polyesters, and vinyl esters, and,thermoplastic resins including nylons, polyethylene, and polypropylene.

The reinforcing rod of the present invention that comprises meso-rodsembedded in a polymer matrix has also got significant utility as astructural member, especially for applications under tension.

In drawings which illustrate the present invention by way of example:

FIG. 1 is a typical tensile stress-strain curves for steel and fibrecomposites;

FIG. 2 is a typical load-displacement curve of a prior art hybrid FRPre-bar;

FIG. 3 a is a side cross-sectional view of a first construction of afirst embodiment of the present invention.

FIG. 3 b is a side cross-sectional enlarged view of a secondconstruction of the first embodiment of the present invention;

FIG. 3 c is a side cross-section enlarged view of a third constructionof the first embodiment of the present invention;

FIG. 3 d is an external side view of the construction of FIG. 3 c, in acommercially practical form;

FIG. 3 e is an external side view of the construction of FIG. 3 c is analternate commercially practical form;

FIGS. 4 a and 4 b are longitudinal and transverse schematiccross-sectional views, respectively of a meso-rod composite re-baraccording to a second embodiment of the present invention; and FIGS. 4 cand 4 d are detail cross sections through line c-c in FIG. 4 a of twopreferred embodiments of meso-rod construction;

FIGS. 5 a, 5 b, and 5 c, respectively are schematics of a innerrod/over-wrap pull-out test, over-wrap/potting resin pull-out test andover-wrap/concrete pull-out test;

FIG. 5 d is the schematic of a typical pull-out test;

FIG. 6 is a load-displacement curve for the inner rod/over-wrap pull-outtest shown schematically in FIG. 4 a;

FIG. 7 are frictional load-displacement curves for the three tests shownschematically in FIG. 4 a, 4 b and 4 c;

FIGS. 8 a and 8 b are two schematics of failure mechanisms;

FIGS. 9 a and 9 b are load-displacement plots for examples embodying thepresent invention to a lesser and greater extent;

FIGS. 10 a and 10 b are side cross-sectional schematic views of a singlemeso-rod and a meso-rod pull-out test;

FIG. 11 load displacement curves for three meso-rod specimens.

Referring now to FIGS. 5 a to 5 d, in preparatory investigations leadingto the development of the present invention, for a specific set ofmanufacturing parameters and materials, the interfacial frictional shearstress after original bond failure of the inner rod/over-wrap interfacewas estimated to be approximately 10 MPa. As part of a failureinvestigation undertaken, the interfacial frictional shear stress afteroriginal bond failure of all of the appropriate interfaces for thechosen manufacturing parameters, materials, and surface preparation,were determined. FIG. 5 d shows a schematic of a typical pullout test.The dimensions for the specific pull-out tests between the over-wrap andthe inner rod, the over-wrap and the potting resin, over-wrap andconcrete are shown respectively in FIGS. 5 a, 5 b, and 5 c. As shown inFIG. 5 a, the over-wrap was cut and the outer surface of the over-wrapwas abraded to ensure the proper interface failure. The end of the rodwas coated with a silicone release agent to remove that contributionfrom the load measurement. The load-displacement curve is given in FIG.6. After initial bond failure along the embedded length, the load due tofriction at the interface decreases as the embedded length decreases.With reference to FIG. 5 d, the interfacial frictional shear stress iscalculated using the following relationship:

$\begin{matrix}{\tau = \frac{P}{\pi \; \varphi \; \left( {l - \Delta} \right)}} & (6)\end{matrix}$

Where (l-Δ) is the embedded length.

Appropriate embedded lengths were selected in order to obtain thedesired failure during pullout. The frictional sliding part of theload-displacement curves (based on the dimensions given in FIGS. 5 a, 5b and 5 c for the inner rod/over-wrap interface, over-wrap/potting resininterface, and over-wrap/concrete interface are given in FIG. 7 forcomparison purposes. An average frictional shear stress for the innerrod/over-wrap interface of 9.6 MPa was determined. The frictionalinterface stress of the over-wrap to potting resin interface was foundto be 7.4 MPa. The final interface was that between the over-wrap andconcrete. For this interface the average shear stress was found to be6.8 MPa. The ordering of the magnitudes of the tensile loads due to thefrictional shear stresses is correct in that the over-wrap to concreteand the over-wrap to potting resin stresses are greater than for theinner rod to the over-wrap. These values confirmed the typical failuremodes of over-wrapped FRP re-bars as well as the potting length of gripsspecified for testing of FRP re-bars.

According to a first embodiment of the present invention, fibrecomposite re-bars were designed, fabricated and tested in order tovalidate the proposed novel pseudo-ductile FRP re-bar. One variant ofthese prototypes is shown in FIG. 3 a.

The selection of materials for the inner rod 1 and over-wrap 2 is amatter of choice for one skilled in the art, given the teaching of thepresent invention. However, the inner rod will generally be selectedfrom carbon fibre/polymer matrix composite, glass fibre/polymer matrixcomposite, or aramid fibre/polymer matrix composite or monolithicpolymer. The fibre over-wrap 2 will generally be of the same choice ofmaterials as the inner rod. The polymer matrix could be a thermosettingpolymer such as epoxy resin, polyester resin or vinyl ester resin or athermoplastic resin such as nylon, polyethylene or polypropylene. Themonolithic polymer would typically be a thermoplastic polymer. Theover-wrap is removed for instance by mechanical cutting (or simply bynot having been applied) at spaced apart locations 3 separated by lengthL. Calculation of L is explained below.

One major issue related to the tensile testing of the re-bars was thechoice of gauge length of the specimens. It was suspected that theunbonded length used in many standards (˜500 mm.) was not representativeof the situation in cracked concrete where a typical crack would benoticeable at about 0.5 mm and could grow for the case-of steelreinforcement to a width of about 50 mm. The various standard testspecifications call for a minimum embedded length in the testing gripsof approximately 250 mm in order to ensure re-bar tensile failure in theunbonded section and not shear failure in the grips.

Tensile testing of the prototype re-bar specimens showed two distinctivetypes of failures. Schematics based on longitudinal slitting of thefailed prototype specimens after testing are shown in FIGS. 8 a and 8 b.The first type pertains to the first examples where the inner rod failedafter sliding over a length with respect to the over-wrap. This is shownschematically in FIG. 8 a. The frictional shear force provided by theinterface in this case was gauged to be comparable to the tensile forcecapability of the inner rod. The second type of failure pertains to thesecond set of prototypes where the over-wrap had breaks in it, thusreducing the frictional shear force between the inner rod and theover-wrap in comparison to the inner rod tensile force capability. Inthese prototypes the inner rod did not break, it continued to slide outof the over-wrap until the test was stopped. This is shown schematicallyin FIG. 8 b. The load-displacement plots showing the two types ofprototype failures for gauge lengths of 50 mm (typical of a large crackwidth) and 0.5 mm (typical of a small crack

$\begin{matrix}{L_{c_{o}} \leq \frac{\sigma_{ur}r_{r}}{\tau_{r}}} & (7)\end{matrix}$

width) are presented in FIGS. 9 a and 9 b respectively. All the plotsexhibit jagged load variations associated with the inner rod sliding outof the over-wrap. This phenomenon is attributed to friction (dry orstatic friction) between the sliding surfaces.

In the example of a preferred embodiment illustrated in FIGS. 3 a and 3b, then, the length L_(c) _(o) of sections of over-wrap that areseparated by serrations or other weakened sections will satisfy theequation:

The lengths of structurally complete sections of over-wrap can beseparated by annular cuts, spiral cuts, chemical abrading, or any othermeans selected by one skilled in the art.

A preferred method of isolating structurally complete sections ofover-wrap, eg. braided over-wrap, is shown in FIG. 3 b. In the re-barshown in FIG. 3 b, the core 1 is made from a fibre/polymer matrixcomposite, and the over-wrap 2 is braided. However, at locations spacedapart by length L calculated as above, along the length of the core, thecore is wrapped with polytetrafluoroethylene (Teflon) tape 11, so thatthere is no adhesion to the inner rod by the over-wrap at those spacedapart locations. Therefore, frictional shear stress at those locationswill be essentially zero.

The pseudo-ductile performance of the FIG. 3 a and FIG. 3 b re-bar willbe virtually identical. That is, local cracks in concrete will tend tocause original bond failure between the over-wrap and the inner-rod indiscrete sections of over-wrap of length L adjacent the crack. At thespaced apart weakened locations 3/11, the over-wrap will break, but theinner-rod will remain intact. Increases in load at the crack site, eg.in the case of an earthquake, may cause further structurally discreteportions of over-wrap to debond from the core, in a pattern radiatingaway from the crack. Until complete failure, though, the re-bar willremain bonded to the concrete at regions away from the cracked region,and even after failure of the bond between the over-wrap and inner rodalong the entire length of the inner rod will resist collapse because ofthe friction between the unbonded over-wrap and the inner rod.

As an example of a design case a high ductility 11.5 mm FRP re-bar witha single inner rod with a diameter of 9.5 mm and a 1 mm over-wrap,assume that the mechanical properties of the designed re-bar is requiredto match those of a standard steel re-bar (i.e. elastic modulus E=200GPa, yield strength σ_(y)=600 MPa, and a very high local ductility atlocal cracks). The inner rod will be fabricated using carbon fibre in amatrix of typical epoxy resin (E=3.5 GPa, and (σ_(m)=100 MPa).

In order to determine the type of carbon fibre to use in order toachieve a re-bar with elastic modulus E_(r)=200 GPa (matching that ofsteel), assume that there is no contribution to the elastic modulus fromthe over-wrap (typically less than 10%), and that the fibre volumefraction in the inner rod is between 50 and 65% (typical range for manyunidirectional fibre composite components).

Since the elastic modulus in primarily a linear function of the elasticmodulus times the fibre volume fraction, the range of elastic moduli ofthe fibres is E_(f)=300 to 400 GPa. For a typical carbon fibre ofapproximately E_(f)=300 GPa (Toryaca M30) the exact volume fractionincluding the contribution from the matrix is calculated using the- Ruleof Mixtures method:

E _(c) =E _(f) V _(f) +E _(m)(1−V _(f))  (8)

Substituting for E_(c), E_(f), E_(m) in the above equation, the volumefraction of the fibre in the composite is found to be V_(f)=0.66.

Again, using the Rule of Mixtures method the tensile strength of thecarbon fibre/epoxy re-bar can be obtained as follows:

σ_(c)=σ_(f) V _(f)+σ′_(m)(1−V _(f))  (9)

where σ_(m)′ is the matrix at fibre failure (˜100 MPa)

Substituting for of σ_(f), σ_(m)′ and V_(f) in the above equation, thetensile strength of the FRP re-bar is found to be 2674 MPa, orapproximately 4.5 times the design value of 600 MPa, thus it will notfail in tension prior to sliding at the interface.

An over-wrap length less than the critical length calculated using thefollowing equation will result in shear failure (sliding against africtional shear stress) at the interface between the inner FRP rod andthe over-wrap. This mode of failure is the desired one, as compared toinner rod failure in tension.

$\begin{matrix}{L_{c_{o}} = \frac{\sigma_{r}^{\prime}r_{r}}{\tau_{r}}} & (10)\end{matrix}$

In the above equation, τ_(r) is found experimentally. For the materials,the manufacturing, and the curing methods used to produce the sampleprototypes, τ_(r) is found to be 9.6 MPa. Substituting this value andthose for σ_(r) and r, the critical length L_(c) is found to be 1.32 m.

A 9.5 mm diameter FRP rod with a pseudo-yield stress of σ_(y)=600 MPashould have a load carrying capacity (σ_(y)π_(r) ²) of 42508 N withinthe elastic regime. Beyond that load, the rod should exhibit a ductilebehaviour, in this case by the sliding of the over-wrap relative to theFRP inner rod. That is to say that the shear load between the FRP innerrod and the over-wrap should be able to withstand a tensile load of 42508 N. This shear load is given by:

Shear load=2πr_(r)lτ_(r)

Substituting for the shear load, r_(r), τ_(r) in the above equation,gives an over-wrap length of l=0.15 m that can carry the shear loadbefore shear failure between the over-wrap and the FRP inner rod takesplace. This is less than the calculated critical length of L_(c), so itwill exhibit the desired pseudo-ductile behaviour.

Thus the high ductility rod will have discontinuity in the over-wrapwith the over-wrap segments having lengths of 0.15 m each.

The second preferred embodiment of the present invention involves theuse of aligned meso-rods, so called because of their intermediate size.

The initial work on this concept focussed on using model specimens inpullout tests. As in the previous concept, control of the interfacialfrictional shear stress between the sliding surfaces is of utmostimportance. In this case however, because of the size and number of themeso-rods, their homogeneity of size and surface consistency isparamount. As proof of concept, ground, and dimensionally accurate steeldowel pins were used. These were embedded in vacuumed epoxy resin. Theresin was cured at room temperature for one day; a completed specimen isshown in FIG. 10 a. Since the resin was relatively transparent it wasalso possible to confirm the fundamental concept of the approach in thatthe initial bond failure occurred at the ends of the meso-rod and thenprogressed towards its centre. Once the original bond had failed allalong the length, one-half of the meso-rod started to pullout of thematrix socket against the frictional shear stress (FIG. 10 b). Theload-extension curves are shown in FIG. 11 for three specimens(specimens 2, 3, & 4). The frictional shear stress develops due to thecontraction of the resin around the steel rods as a result of chemicalshrinkage of the resin during polymerization. The results for the threespecimens are very similar indicating good repeatability between castingruns. The results are also similar in nature to those obtained from thepull-out tests shown in FIG. 7. Also included in FIG. 11 is the curvefor a specimen in which an elevated temperature epoxy resin (curetemperature 110 degrees C.) was used. It is clear that, as expected,additional frictional shear stress results from the resin thermallyshrinking around the dowel pin. Other values of frictional shear stresscan be obtained using other resin types and other cure schedules. Thus,it is possible to obtain the desired failure of frictional shear stress(the most critical parameter) for the application.

A full-size re-bar 4 incorporating meso-rods 5 consists of a number offibre composite meso-rods (multiple meso-rods), staggered along thelength of the re-bar, encapsulated in a second polymer matrix 6 as shownin FIGS. 4 a and 4 b. The individual meso-rods could also be continuousrods that are almost completely cut through. Two different ways toprovide continuous rods that are almost cut through are shown in FIGS. 4c and 4 d. The small amount of continuous fibre composite which can belocated at any point in the cross-section aids in aligning the meso-rodsalong the axis of the re-bar during the manufacturing process. Tensilefailure will occur at the reduced cross-section points at low values oftensile load. Due to the reduced elastic modulus magnitudes indiscontinuous -fibre composites, it is desirable to have some continuousfibre composite material along the entire length of the re-bar. This maybe provided by the continuous composite referred to previously.

The following is an example of a high ductility composite multiplemeso-rod re-bar, 11.5 mm outside diameter, with properties similar tothose described above in relation to FRP inner rod/over-wrap re-bardiscussed in the previous embodiment, that is, an elastic modulus in theE=300 GPa range, yield strength in the τ=600 MPa range, and high localductility.

The re-bar uses an epoxy matrix with an elastic modulus of E=3.5 GPa,the stress σ_(m)′ in the matrix at fibre failure being 100 MPa.

Since elastic modulus will be reduced due to the use of discontinuousmeso-rods as compared to continuous meso-rods and elastic modulusincrease with fibre volume fraction, a fibre volume fraction at the highend of the practical range for manufacturing will be chose, namelyV_(f)=0.65. This is accomplished with 22 meso-rods, each of 2 mmdiameter at any cross- section. Again, in order to maximise the elasticmodulus of the individual meso-rods, a high modulus carbon fibre shouldbe selected. For example, Torayca M40, with E_(f)=400 GPa and σ_(f)=700MPa along with a high fibre volume fraction within the meso-rod, namelyV_(f)=0.65.

Since the re-bar is to carry the same load (i.e., design capacity) asthe inner rod with over-wrap concept, i.e. 42508 N, the required loadcapacity per meso-rod is 1932 N.

For a constant frictional shear stress, τ_(r), along the length of themeso-rod, the load in the meso-rod increases linearly from the end. Fora load of 1932 N to be carried over one-half the length of the meso-rod,the load at mid-point of the meso-rod must be twice the average value,i.e., 3864 N.

The length of the meso-rod is calculated as follows:

$\begin{matrix}{3864 = {2\; \pi \; {r_{m}\left( {l_{m}/2} \right)}\tau_{m}}} \\{= {2\; {\pi \left( {1 \times 10^{- 3}} \right)}\left( {l_{m}/2} \right)\tau_{m}}} \\{l_{m} = {0.12\mspace{11mu} m}}\end{matrix}$

Where τ_(m) was measured experimentally. Thus, 22 meso-rods of length0.12 m are required to provide the load capability of 42,508 N.

In order to ascertain that individual meso-rods do not fail in tensionbefore failing in shear, assume that the ultimate strength of meso-rodin tension:

$\begin{matrix}{\sigma_{c} = {{\sigma_{f}V_{f}} + {\sigma_{m}^{\prime}\left( {1 - V_{f}} \right)}}} \\{= {{\left( {1700 \times 10^{6}} \right)(0.65)} + {\left( {100 \times 10^{6}} \right)\left( {1 - 0.65} \right)}}} \\{= {1140\mspace{14mu} {MPa}}}\end{matrix}$

Load in meso-rod at ultimate strength:

$\begin{matrix}{L = {\sigma_{c}\left( A_{{meso} - {rod}} \right)}} \\{= {\left( {1140 \times 10^{6}} \right)\left( {\pi/4} \right)\left( {2.0 \times 10^{- 3}} \right)^{2}}} \\{= {3580\mspace{14mu} N}}\end{matrix}$

This is much larger than the load at which the interface sliding willtake place, i.e., 1932 N, therefore, the meso-rods will not fail intension prior to interfacial sliding.

Finally, the elastic modulus of the re-bar with multiple meso-rods canbe calculated using an accepted formula (Halpin-Tsai) for the elasticmodulus of discontinuous fibre composites. As noted above, the elasticmodulus is a linear function of the elastic modulus times the volume,for each constituent. In the present case, with E=3.5 GPa for the epoxy(volume fraction of 35%) and E=400 GPa for Torayca M40 (volume fractionof 65%), the overall elastic modulus will be 216 GPa which is close tothe desired value of 200 GPa. Exact values of elastic modulus can beachieved by altering the fibre volume fraction.

The pseudo-ductility concepts of re-bars proposed here can also beconceived through a number of alternate designs other than those shownin FIGS. 3 and 4. Any arrangement that provides for a controlled andgauged frictional shear stress between a medium anchored to the concreteand an inner rod that can sustain tensile loading would work. In thecase of the arrangement shown in FIG. 3 a the inner rod is anchored tothe concrete by the braided over-wrap fibre bundles while braiding,using a different type of resin (whether thermoplastic or thermoset), orthrough surface preparation of the inner rod. In a similar manner, thecontrol of the frictional shear load between the meso-rods and thesurrounding matrix can be achieved by changing the material of themeso-rods, the surrounding matrix and its cure schedule, as well as bythe surface preparation of the meso-rods.

For the case of the single FRP inner rod, the tensile force capabilityof the rod must be higher than the ultimate tensile force required,while the frictional shear force capability between that inner rod andthe segments of the over-wrap must be gauged to be at the tensile loadfor the yield strength required. When the load at a section of thepseudo-ductile re-bar exceeds the yield load, sliding occurs, thusproviding the pseudo-ductility effect. This is the case portrayed inFIG. 8 b. If the frictional shear force capability between that rod andthe segments of the over-wrap is close to the ultimate tensile forcecapability of the single inner rod, the case shown in FIG. 8 a mayoccur.

The form of construction shown in FIGS. 3 c, 3 d and 3 e provides analternative pseudoductile re-bar that has a construction similar to thatshown in FIG. 3 a, and especially 3 b, but performance characteristicssimilar to the product shown in FIG. 4 a. In the FIGS. 3 c, 3 d and 3 eproduct, an inner rod 1 similar to that shown in FIGS. 3 a and 3 b,typically a carbon fibre rod is textured, typically by the provision ofa Kevlar over-wrap 2, and cured to provide a finished rod having desiredelastic modulus and yield strength. The wrap, however, is not dividedinto structurally isolated sections. Rather, a further layer 12 of amaterial such as polyurethane foam, cardboard, or other sheet materialis wrapped over the outer layer of the rod, dividing it into discretesections that will adhere to a concrete matrix, where the Kevlarover-wrap is exposed, and other sections where there will be no bondbetween the sheet material over-wrap 12 and the Kevlar over-wrap 2.Accordingly, when the re-bar is subject to high frictional shear stress,there will be no inner rod failure or Kevlar over-wrap failure; rather,at a predetermined level of stress, the rod will tend to slide in theconcrete—much like the sliding action of the meso rods in the FIG. 4 aembodiment of the present invention.

As can be seen from FIGS. 3 c and 3 d, typically bands of predeterminedwidth of sheet material are removed, either in the factory or on site,depending on the length of the re-bar, and the desired degree of yieldstrength. In order to make removal of the correct amount of sheetmaterial simple at a job site, bands may be colour, number or lettercoded, as shown in FIG. 3 d. Alternatively, sheet material may beremoved from the re-bar in lengthwise running strips, as shown in FIG. 3e. Where it is desired to have material removable at a job site, thematerial should be provided on its inner surface with an adhesive thatcan be peeled away fully so that the Kevlar over-wrap is not fouled.Also, the material should be perforated along lines 13 in predeterminedpatterns, to permit it to be peeled off easily.

Moreover, it will be appreciated that an effect similar to that obtainedin the FIGS. 3 c, 3 d and 3 d constructions may be obtained byselectively texturing a rod, by selectively sanding it in “patches”during fabrication, or by embossing a rod during fabrication in apredetermined pattern, eg. over only a half or a third of itscircumferential area.

It should be emphasised at this point that while braiding was used toproduce the over-wrap, other means can also be utilized. Wrapping ofvarious types of strips on an existing inner rod is one such approach.Furthermore, while a serrated over-wrap was used to limit theinterfacial frictional shear force at a segment of the inner rod, othermeans like a helical wrap would produce a similar effect.

The primary use of the reinforcing rod of the present invention will bein reinforcing concrete structures, where it will take the place ofsteel. Other uses will be obvious to one skilled in the art, and includereinforcement of mine tunnel and stope ceiling and walls, especially incorrosive environments, post tensioning of lightweight beams,fabrication of automotive and rolling stock chassis, airframes and thelike. It will be understood, moreover, that the large majority ofalternative uses relate to the structurally discontinuous meso-rodcontaining embodiments of the present invention, since they do not relyon adhesion between the outer surface of the rod and a surroundingenvironment to exhibit pseudo-ductility.

Moreover, it will be understood that the rod of the present inventionneed not be circular in cross-section. The present invention may be inthe shape of other traditional structural elements, such as elliptical,I-shapes, T-shapes, L-shapes, U-shapes, box-shapes. It is also withinthe scope of the present invention to utilize structurally orfunctionally discontinuous meso-rods, for instance, in a particular zoneof a structural element. For example, it is within the scope to thepresent invention to embed a plurality of structurally discontinuousmeso-rods in the base of an extruded aluminum I-beam, therebystrengthening same, and providing a measure of pseudo-ductility to same.

Moreover, it will be understood that an additional application is inincreasing the toughness of structures where toughness is measured asthe work done (energy absorption) in separating two or more parts of astructure.

1. A re-bar (reinforcing rod) for concrete comprising an inner rod of afirst material, and an over-wrap of a second material, said over-wrapbeing structurally or functionally discontinuous relative to said innerrod.
 2. A reinforcing rod as claimed in claim 1, wherein said inner rodcomprises a rod made from a material consisting of a polymeric materialor a polymeric matrix and reinforcing fibre.
 3. A reinforcing rod asclaimed in claim 1, wherein said over-wrap is a polymeric material or afibrous material set in a polymeric matrix.
 4. A reinforcing rod asclaimed in claim 3, wherein said fibrous material is selected from thegroup consisting of ceramic materials including carbon and glass fibres,polymeric materials such as aramid and polyethylene, and metallicmaterials like steel.
 5. A reinforcing rod as claimed in claim 4,wherein said resin is selected from the group consisting of thermosetresins such as epoxies, polyesters, and vinyl esters, and thermoplasticresins such as nylon, polyethylene, and polypropylene.
 6. A reinforcingrod as claimed in claim 3, wherein said over-wrap includes zones ofweakness separating full strength lengths of said outer layers.
 7. Areinforcing rod as claimed in claim 3, wherein said over-wrap includeszones of low frictional shear stress between the over-wrap and the innerrod interspersed among high frictional shear stress zones.
 8. Areinforcing rod as claimed in claim 7 wherein said low frictional shearstress zones are achieved by application of a layer of low frictionmaterial on said inner rod at said zones of low frictional shear stress,and said over-wrap covers said low frictional material.
 9. A reinforcingrod as claimed in claim 6 wherein said zones of weakness are formed bymechanically removing a portion of said over-wrap after it has beenapplied to said inner rod.
 10. A reinforcing rod as claimed in claim 6,wherein said zones of weakness are defined by short spaced apart lengthsof said inner rod having no outer-wrap over same.
 11. A reinforcing rodas claimed in claim 6 wherein said zones of weakness are defined bylocal shearing of polymer over-wrap.
 12. A reinforcing rod as claimed inclaim 1, wherein said inner rod is a cylindrical rod having radius r_(r)and an ultimate tensile strength σ_(ur), the frictional shear stressafter bond failure between the inner rod and the over-wrap is τ_(r), andsaid over-wrap is comprised of structurally discontinuous portionshaving a maximum length L_(c) _(o) , wherein$L_{c_{o}} = \frac{\sigma_{ur}r_{r}}{\tau_{r}}$
 13. A rod as claimed inclaim 12, wherein said radius r is in the range of 1-30 mm.
 14. A rod asclaimed in claim 12, wherein said length L_(c) _(o) is in the range of1-150 cm.
 15. A rod as claimed in claim 13, wherein said radius r is inthe range of 3-8 mm.
 16. A rod as claimed in claim 15, wherein saidradius r is in the range of 4-6 mm.
 17. A rod as claimed in claim 16,wherein said radius r is in the range of 4-5 mm.
 18. A rod as claimed inclaim 17, wherein said radius r is 4.5 mm.
 19. A rod as claimed in claim13, wherein said length L is in the range of 10-20 cm.
 20. A rod asclaimed in claim 19, wherein said length L is in the range of 12-18 cm.21. A rod as claimed in 20, wherein said length L is about 15 cm.
 22. Amethod of inducing pseudo-ductility or toughness in a fibre reinforcedcomposite rod, said rod comprising a solid core and a fibre reinforcedpolymeric resin over-wrap on said core, said method comprisingstructurally interrupting said over-wrap at spaced apart locations. 23.A method as claimed in claim 22, wherein said over-wrap is applied as aresin impregnated fibre braid.
 24. A method as claimed in claim 22,wherein said over-wrap is applied as a resin impregnated fibre yarn,unidirectional tape or woven fabric tape helically wound on said core.25. A method as claimed in claim 22, wherein said over-wrap isstructurally interrupted by being cut in spaced apart annular rings. 26.A method as claimed in claim 22, wherein said over-wrap is structurallyinterrupted by being cut in a continuous helical pattern.
 27. A methodas claimed in claim 22, comprising the steps of i) providing an innerrod comprising solid core a fibre reinforced polymer ii) applying bandsof material having low frictional shear stress at spaced apart locationson said solid core iii) applying a fibre reinforced polymeric resinover-wrap over the banded core, whereby said bands of low frictionalshear stress material structurally separate zones of over-wrap bonded tosaid core.
 28. A method as claimed in claim 22, wherein said solid coreis a cylindrical rod having radius r_(r) and an ultimate tensilestrength σ_(ur), the frictional shear stress after bond failure betweenthe solid core and the over-wrap is τ_(r), and said over-wrap iscomprised of structurally discontinuous portions having a maximum lengthL_(c) _(o) , wherein $L_{c_{o}} = \frac{\sigma_{ur}r_{r}}{\tau_{r}}$29. A method as claimed in claim 28, wherein said radius r is in therange of 1-30 mm.
 30. A method as claimed in claim 28, wherein saidlength L_(c) _(o) is in the range of 1-150 cm.
 31. A method as claimedin claim 29, wherein said radius r is in the range of 3-8 mm.
 32. Amethod as claimed in claim 31, wherein said radius r is in the range of4 -6 mm.
 33. A method as claimed in claim 32, wherein said radius r isin the range of 4 -5 mm.
 34. A method as claimed in claim 33, whereinsaid radius r is 4.5 mm.
 35. A method as claimed in claim 29, whereinsaid length L is in the range of 10-20 cm.
 36. A method as claimed inclaim 35, wherein said length L is in the range of 12-18 cm.
 37. Amethod as claimed in 36, wherein said length L is about 15 cm.
 38. Areinforcing rod comprising a composite of at least two materials, atleast one of which is present in structurally discontinuous lengths. 39.A reinforcing rod as claimed in claim 38, comprising at least threematerials, at least one of which is present in structurally orfunctionally discontinuous lengths.
 40. A reinforcing rod as claimed inclaim 38, wherein said composite comprises a polymer matrix havingembedded therein structurally discrete meso-rods of length L_(c) _(o)with radius r_(m), ultimate and tensile strength σ_(um), the frictionalshear stress between a meso-rod and the polymer matrix being representedby τ_(m), wherein $L_{c_{m}} = \frac{\sigma_{um}r_{m}}{\tau_{m}}$
 41. Areinforcing rod as claimed in claim 40, wherein said structurallydiscrete meso-rods comprise a plurality of aligned meso-rods that are,axially, substantially randomly distributed.
 42. A reinforcing rod asclaimed in claim 40, wherein said structurally discrete meso-rodscomprise a plurality of elongated meso-rods breakable by a tensile loadsubstantially less than the ultimate tensile load of each meso-rod, atpredetermined weakened locations that are randomly staggered, from rodto rod.
 43. A reinforcing rod as claimed in claim 40, wherein L_(c) _(o)is in the range of 5-30 cm.
 44. A reinforcing rod as claimed in claim43, wherein L_(c) _(o) is in the range of 5-25 cm.
 45. A reinforcing rodas claimed in claim 43, wherein L_(c) _(o) is in the range of 8-20 cm.46. A reinforcing rod as claimed in claim 43, wherein L_(c) _(o) is inthe range of 10-15 cm.
 47. A reinforcing rod as claimed in claim 43,wherein L_(c) _(o) in the range of 11-13 cm.
 48. A reinforcing rod asclaimed in claim 43, wherein L_(c) _(o) is optimally about 12 cm.
 49. Areinforcing rod as claimed in claim 43, wherein r_(m) is in the range of0.5-4.0 mm.
 50. A reinforcing rod as claimed in claim 43, wherein r_(m)is in the range of 0.5-3.0 mm.
 51. A reinforcing rod as claimed in claim43, wherein r_(m) is in the range of 1.0-3.0 mm.
 52. A reinforcing rodas claimed in claim 43, wherein r_(m) is in the range of 1.5-2.5 mm. 53.A reinforcing rod as claimed in claim 43, wherein r_(m) is about 2.0 mm.54. A reinforcing rod as claimed in claim 40, wherein said meso-rods aremade from a material selected from the group consisting of ceramicmaterials including carbon fibres and glass fibres.
 55. A reinforcingrod as claimed in claim 54, wherein said polymer matrix is selected fromthe group consisting of thermoset resins including epoxies, polyesters,and vinyl esters, and thermoplastic resins including nylons,polyethylene, and polypropylene.
 56. A structural rod comprising acomposite of at least two materials, at least one of which is present instructurally discontinuous lengths.
 57. A structural rod as claimed inclaim 56, comprising at least three materials, at least one of which ispresent in structurally or functionally discontinuous lengths.
 58. Astructural rod as claimed in claim 56, wherein said composite comprisesa polymer matrix having embedded therein structurally discrete meso-rodsof length L_(c) _(o) with radius r_(m), ultimate and tensile strengthσ_(um), the frictional shear stress between a meso-rod and the polymermatrix being represented by τ_(m), wherein$L_{c_{m}} \leq \frac{\sigma_{um}r_{m}}{\tau_{m}}$
 59. A structural rodas claimed in claim 58, wherein said structurally discrete meso-rodscomprise a plurality of aligned meso-rods that are, axially,substantially randomly distributed.
 60. A structural rod as claimed inclaim 58, wherein said structurally discrete meso-rods comprise aplurality of elongated meso-rods breakable by a tensile loadsubstantially less than the ultimate tensile load of each meso-rod, atpredetermined weakened locations that are randomly staggered, from rodto rod.
 61. A structural rod as claimed in claim 58, wherein L_(c) _(o)is in the range of 5-30 cm.
 62. A structural rod as claimed in claim 61,wherein L_(c) _(o) is in the range of 5-25 cm.
 63. A structural rod asclaimed in claim 61, wherein L_(c) _(o) is in the range of 8-20 cm. 64.A structural rod as claimed in claim 61, wherein L_(c) _(o) is in therange of 10-15 cm.
 65. A structural rod as claimed in claim 61, whereinL_(c) _(o) is in the range of 11-13 cm.
 66. A structural rod as claimedin claim 61, wherein L_(c) _(o) is in the range of 12 cm.
 67. Astructural rod as claimed in claim 61, wherein r_(m) is in the range of0.5-4.0 mm.
 68. A structural rod as claimed in claim 61, wherein r_(m)is in the range of 0.5-3.0 mm.
 69. A structural rod as claimed in claim61, wherein r_(m) is in the range of 1.0-3.0 mm.
 70. A structural rod asclaimed in claim 61, wherein r_(m) is in the range of 1.5-2.5 mm.
 71. Astructural rod as claimed in claim 61, wherein r_(m) is about 2.0 mm.72. A structural rod as claimed in claim 58, wherein said meso-rods aremade from a material selected from the group consisting of ceramicmaterials including carbon fibres and glass fibres.
 73. A structural rodas claimed in claim 72, wherein said polymer matrix is selected from thegroup consisting of thermoset resins including epoxies, polyesters, andvinyl esters, and thermoplastic resins including nylons, polyethylene,and polypropylene.
 74. A reinforcing or structural rod as claimed inclaim 38, having a cross-section that is of a shape selected from thegroup consisting of circular, elliptical, oval, square, rectangular,triangular, diamond shapes, dog-bone shaped, L-shaped, T-shaped,U-shaped, and 5-20 sided polygon shaped.
 75. A method of inducingtoughness in a structural element, comprising embedding in saidstructural element a plurality of structurally or functionally discretemeso-rods.
 76. A reinforcing rod comprising a composite rod having aninner core and an outer surface, said outer surface being textured overa predetermined portion thereof to mechanically grip a concrete matrixin which a said rod is embedded.
 77. A rod as claimed in claim 76,wherein said entire outer surface is textured, and portions thereof aremasked from contact with a said concrete matrix by the provision of asheet material thereon.
 78. A rod as claimed in claim 77, wherein saidsheet material is provided over said entire outer surface, and isselectively removable.
 79. A rod as claimed in claim 78, wherein saidsheet material is provided with circumferential bands of perforations,to permit portions thereof to be removed without damaging other portionsthereof.
 80. A rod as claimed in claim 78, wherein said sheet materialis provided in longitudinally extending strips separated by longitudinallines of perforations to permit selected strips to be removed withoutdamaging others.
 81. A rod as claimed in claim 79, wherein said sheetmaterial is provided with indicia thereon to assist in theidentification of portions to be removed.
 82. A rod as claimed in claim76, wherein said outer surface is textured by the provision of atextured outer wrap.
 83. A rod as claimed in claim 82, wherein saidouter wrap is a polyaramide Kevlar™ outer wrap.